1. Field of the Invention
This invention relates generally to electronic circuits for the controlled switching of high current loads and more particularly relates to protecting solid state, smart, highside, high current, power switches from damage caused by excessively high currents that begin essentially instantaneously, such as from a short across the load terminals, and destroy the smart power switches, despite their associated protection circuits.
2. Description of the Related Art
Convenient energy efficient operation of electrical equipment, appliances and other electrical devices often requires a switch for turning the device on and off. This is particularly important when multiple different devices are powered by a vehicle battery in order to minimize drain on the battery by permitting selected operation of only those devices that are currently being used. Some electrical devices are high power devices that draw large currents. For those, it is often desirable to control the switch that switches the high current from a low power electrical signal. For this purpose, relays or power contactors were traditionally used. However, such devices have mechanical electrical contacts which are subject to corrosion and the possibility of having the contacts welded together and suffer from numerous other failure modes.
When solid state technology appeared, it was first applied to develop solid state switches that could be used for switching low currents associated with low power applications but the available solid state switching devices could not tolerate higher currents. However, MOS/FET devices have more recently been developed that can switch currents on the order of a hundred amperes or more. These switching devices have thousands of MOS/FETs formed in an integrated circuit and connected in parallel so they each carry a small portion of the current and operate together as a high current, composite MOS/FET. Additionally, modern integrated circuit technology also permits a variety of other circuits to be formed in the same integrated circuit to provide for operating, controlling and protecting the MOS/FETs. Because these associated circuits include logic and sensing circuits and can detect a variety of fault conditions and turn off the MOS/FETs in order to protect the integrated circuit module, the modules are called “smart”. Consequently, the combination of the current switching MOS/FETs and their associated circuits provide high power switch modules that manufacturers can use to construct high power switches for applications in their field. In addition to the relatively high currents that each integrated circuit module can switch, manufacturers of high power switches can connect multiples of these integrated circuit modules in parallel to increase the maximum current their products can switch by a multiplier equal to the number of parallel modules.
A particularly useful type of power switching module is a smart, high side, high current power switch. High side means that their power switching terminals are connected between the high side of the power source, such as the positive terminal of a battery, and the load being switched. The load is connected between the switch module and a common power ground. There are a few manufacturers who supply such modules and FIG. 1 illustrates an example module 10, which is a PROFET® BTS555 offered by Infineon Technologies. This first generation high current power switch module was designed to control a starting motor on a vehicle.
Referring to FIG. 1, The power switch module 10 is an integrated circuit that has a composite MOS/FET 12 and several associated circuits for operating, controlling and protecting the MOS/FET. Typically, such modules have four terminals to which other, external circuit elements are connected. One terminal is a load terminal L that is connected to one terminal of the load being controlled by the smart switch. The other terminal of the load itself is connected to the power circuit ground. The power source, such as a 12 or 24 volt vehicle battery, is connected to a battery terminal B and the other power source terminal is connected to the power circuit ground. Control of the switching module for turning it ON and OFF is accomplished by grounding a third, control input terminal C to turn it ON and disconnecting that ground connection to turn it OFF.
The fourth terminal M is a sensing or diagnostic terminal that is an output of a current mirror circuit. The current mirror circuit utilizes a few of the large number of individual MOS/FETs formed in the integrated circuit of the switch module. A current mirror circuit is well known in the art and, as applied to the switch module, the drains of all the individual MOS/FETs are connected together and most of the sources of the individual MOS/FETs are connected to the main output load terminal L for switching the high load current. A few of the sources, however, are instead used for the current mirror circuit and connected to the sensing terminal M. As known in the art, a current minor circuit provides a current through the terminal M that is proportional to the load current through the majority of MOS/FETs that conduct the load current through the module 10. Preferably, the terminal M is connected to the power circuit ground through a resistor RIS having a resistance, for example, of 1000 ohms. However, the current will have the current proportionality property as described above if a conductor is substituted for the resistor RIS.
Among the protection circuits that manufacturers have integrated into the power switch modules is a circuit that senses module temperature and shorts the MOS/FET gate to ground and thereby open the MOS/FETs to a non-conductive, high resistance state in the event a temperature above a selected maximum operating temperature is sensed. Consequently, if a power switch module gets too hot, it is shut down by its own associated, internal circuitry before the module is damaged. Another protection circuit that is commonly included among the associated protection circuits is a circuit that senses load current and opens the MOS/FETs to a non-conductive, high resistance state in the event a selected maximum operating current is sensed. Yet another associated protection circuit typically integrated in the smart power switch module is a circuit that modulates the conduction of the power switch module. For example, if a reverse voltage is applied to the power switch module, such as from an inductive load immediately after it has been shut off, the power switch module modulates the current so that the dissipation of heat energy from the power switch module can be extended and distributed over a longer time interval. Still another protection circuit ordinarily provided among the control and protection circuits of the power switch module is an undervoltage protection circuit. This circuit senses the output voltage and, if the battery slowly discharges to a level that the control and protection circuits have an insufficient supply voltage to power them, the undervoltage circuit will operate and turn OFF the power switch. The variety of protection circuits in the power switch module include both digital logic and analog circuits.
In addition to protection circuits, there are also control circuits associated with the MOS/FETs in the integrated circuit. It is necessary to develop a charge that is positive to negative between the gate and the source to turn on the MOS/FETs. To do this, since the device is floating, a charge pump circuit is used. The charge pump circuit is well known in the art and gets its power from a drain connection. The MOS/FETs are controlled by supplying a controllable ground connection through module terminal C. When terminal C is not grounded, the internal electronics cause the gate to be shorted to the source turning the MOS/FETs OFF. When terminal C is grounded, the charge pump is turned ON and supplies a high enough voltage between the source and the gate to turn ON the MOS/FETs to their conducting state.
However, unfortunately I have discovered that, despite all the protection and control circuits on board such power switching modules, the switching module are still subject to destruction by a high surge current, such as is caused by a sudden, very low resistance, short across the load terminals. An essentially instantaneous short, such as can be caused by a mechanic inadvertently inserting a metallic tool between the load terminals, causes the destruction of such power switches. I have experimentally replicated the destruction of the power switch modules in a laboratory by such an instantaneous short circuit.
It is therefore an object and feature of the present invention to provide a manner of protecting such integrated power switch modules by means of a circuit that is, of necessity, restricted to connection to two or more of the only four terminals that are externally accessible on the power switch modules. This permits manufacturers, who utilize existing smart, highside, high current, power switch module designs in their products, to protect the power switch modules from destruction by such instantaneous very low resistance short circuits.